Decentralized voting with unconditional privacy

Abstract

The aggregation of conflicting preferences is a key issue in multiagent systems. Due to its universality, voting has a central role among preference aggregation mechanisms. Voting among a set of alternatives can be used for such diverse tasks as choosing a joint plan in a multiagent system, determining a leader in a group of humans or agents, or voting among different resource or task allocations. Maintaining privacy of individuals' votes is crucial in order to guarantee freedom of choice (e.g., lack of vote coercing and reputation effects), and not facilitate strategic voting. We investigate whether unconditional full privacy can be achieved in voting, that is, privacy that relies neither on trusted third parties (or on a certain fraction of the voters being trusted), nor on computational intractability assumptions (such as the hardness of factoring). In particular, we study the existence of distributed protocols that allow voters to jointly determine the outcome of an election without revealing any information but the election outcome. We show the impossibility of reaching unconditional full privacy for a variety of the most common voting schemes ranging from simple veto voting to the single transferable vote scheme. On the positive side, we propose several distributed protocols that privately compute the outcome of common voting schemes while only revealing a limited amount of information.

Large-Scale Election Campaigns: Combinatorial Shift Bribery

“…It is quite natural to consider Combinatorial Shift Bribery also from a different perspective. Instead of asking what happens for a small number of candidates, we might ask about the complexity of Combinatorial Shift Bribery for a small number of voters (see, for example, Brandt, Harrenstein, Kardel, & Seedig, 2013;Chen et al, 2015, for some motivation as to why looking at elections with few voters is interesting). In this case we obtain hardness only for the Borda rule.…”

Analyzing the Privacy of a Vickrey Auction Mechanism

“…The consequence is that a group of n-1 colluders that includes the winner can uncover the global function. Let us note that as stated by Brandt and Sandholm (2004), it is impossible to find a completely private mechanism to perform the Vickrey auction. So, providing a mechanism where even the collusion of n-1 bidders is unable to breach the privacy is not possible.…”

“…Besides, we show that other collusion threats may appear by chance, though the probability of these situations decreases with the number of bidders. As it is stated in Brandt and Sandholm (2004), it is impossible to find a completely private mechanism to perform the Vickrey auction. This result imposes an upper bound of privacy-preserving efficiency in that framework.…”

“…In this paper, we address these issues in a deeper way. Let us note that, as it is stated in Brandt and Sandholm (2004), it is impossible to create a completely private mechanism to perform the Vickrey auction. In particular, we identify a gap between the proposed protocol and the complete privacy: If any n-2 bidders and the winning bidder collude, the privacy is lost.…”

Towards Secure Virtual Elections: Multiparty Computation of Order Based Voting Rules

“…For example, in score-based voting rules, a score for each candidate is computed subject to the specifications of the underlying voting rule, and then the winning candidate is the one whose aggregated score is highest. In this study we focus on order-based (a.k.a pairwise-comparison) voting rules, where the relative order of the candidates is considered by the underlying voting rule [7].…”